There is a need for high reliability with respect to failure and fire, an increased number of possible charge and discharge cycles (increased service life), and longer usage time (higher energy density) from a single charging in secondary batteries that are used for energy storage or for powering mobile devices. The plateau of the voltage per unit battery is the highest during discharge, being 4.1 V in the conventional organic electrolyte battery, and increasing the voltage per unit battery is effective for further increasing the energy density.
The method described below has been proposed for further increasing the discharge voltage. Specifically, a positive electrode material has been synthesized from a material that exhibits a voltage plateau of 4.7 V or higher during charge and discharge, wherein the material is composed of a compound of a positive electrode active material having a so-called spinel-type crystal form (e.g., LiMxMn(2−x)O4, wherein Ni, Co, Fe, or the like is used as the metal M, and the composition ratio x is equal to 0.5). The reason for these effects in the voltage plateau is considered to be that a high voltage is produced by a change in the valence of the metal M ions to another valence brought about by the state of the metal M ions during synthesis.
However, in the conventional organic electrolyte battery, oxidative decomposition of the organic substance was considered to be unavoidable under high voltages of about 4 V or higher, and there was concern that battery performance would suffer as a result of the accumulation of byproducts at the positive electrode/electrolyte interface due to oxidative decomposition of the organic substance used in the electrolyte as charging and discharging were repeated. Besides the change in valence of the metal ions that are naturally expected to react when the positive electrode itself is under high voltage, another concern was irreversible charge compensation due to oxygen desorption, and the suppression of these phenomena was considered necessary for the effective functioning of a high-voltage positive electrode.
The inventors have therefore proposed a secondary battery in which an organic electrolyte is interposed between the positive electrode material and the negative electrode material in the “secondary battery” of Patent Document 1 with the object of providing a secondary battery capable of suppressing oxidative decomposition of the organic electrolyte and desorption of oxygen from the positive electrode active material, wherein the secondary battery is characterized in that an inorganic solid electrolyte film is formed in advance between the positive electrode material and the organic electrolyte.
According to this invention, oxidative decomposition of the organic electrolyte by the positive electrode material of the secondary battery that acts as an oxidizing agent during charging can be suppressed by the inorganic solid electrolyte film formed between the positive electrode material and the organic electrolyte, and desorption of oxygen from the positive electrode material can also be suppressed. The organic electrolyte can therefore be prevented from degrading, and a high voltage during discharge can be maintained for a long time even when charging and discharging are repeated.
FIG. 15 shows a schematic view of the procedure for manufacturing the secondary battery described in Patent Document 1 as a reference.
The positive electrode sheet of this secondary battery is manufactured by a process in which a positive electrode active material (3) is applied on a metal electrode substrate (18) by an electrostatic discharge (ESD) method using an electrostatic discharge device, and the metal electrode substrate is then heated to evaporate the solvent. An inorganic solid electrolyte film (7) having a thickness of about 10 nm and an organic electrolyte (solid polymer electrolyte) film (5) are formed on the positive electrode sheet, and the assembly is pressure bonded to a negative electrode material (4), whereby the secondary battery is formed.    [Patent Document 1] Japanese Laid-open Patent Application 2003-338321